|Covalent Bonding and the
|Hydrogen, with just one electron in its 1s orbital, can achieve
a noble gas configuration (that of helium) by obtaining a share of one electron
from another atom. The Lewis structure of H2 implies that both
atoms have access to both electrons in the bond.
|Since hydrogen obtains a stable valence shell configuration when
it shares just one pair of electrons with another atom, hydrogen atoms only
form one covalent bond. (This should be easily confirmed from the organic
unit as well).
|The Octet Rule|
|The valence shell of the noble gases other than helium all contain
eight electrons, and the tendency of many atoms to acquire such an outer
shell electron configuration forms the basis of the octet rule.
|Octet Rule: When atoms react, they tend to achieve an outer shell
having eight electrons.
|Many of the representative elements (sodium and chlorine for instance)
follow this rule when they form ions. In the ionic case they achieve the
noble gas shell configuration by either a gain or loss of electrons. When
atoms other than hydrogen form covalent bonds, an octet is accomplished by
sharing. The octet rule can be used to explain the number of covalent bonds
an atom forms. This number normally equals the number of electrons that atom
needs to have a total of eight electrons (an octet) in its outer shell. For
example, the halogens (Group VIIA), all have seven valence electrons.
|The Lewis symbol for a typical member of this group, chlorine, is
|There is only one electron needed to complete an octet. Of course,
chlorine can actually gain this electron when it becomes and ion. When chlorine
combines with any other nonmetal, the transfer of electrons is not energetically
favourable. Therefore, in forming such compounds as HCl or Cl2,
chlorine gets the one electron it needs by forming a covalent bond.
H· + :Cl: ------> H:Cl: --> H-Cl:
. .. ..
|The bond produced by the sharing of one pair of electrons between
two atoms is called a single bond. This first bond is designated a sigma
bond. There are however many molecules in which more than one pair of electrons
are shared between two atoms. For example, we that nitrogen, is diatomic,
N2. Each N atom has the Lewis symbol of
|and each nitrogen needs three electrons to complete its octet. When
the N2 molecule is formed, each nitrogen atom shares three electrons
with the other.
|The result is called a triple bond. Notice that in the Lewis formula
for the molecule, the three shared pairs of electrons are placed between
the two atoms. We count all of these electrons as though they belong to both
of the atoms. Each nitrogen therefore has an octet
|In the triple bond above the middle bond is a sigma type bond and
the other two are Pi bonds. The Pi bonds are slightly different because
they have to be bent out of shape in order to connect properly. But more
on this later.
|Double bonds are also possible. eg. CO2
: O :: C :: O : or : O = C = O :
.. .. .. ..